The present invention relates to an apparatus and a method for producing a fingerprint ridge extraction system to accurately enhance and reproduce fingerprint images. The heart of the invention lies in the implementation of a reading technique utilizing a contact image sensor array, with many image enhancement methods to better define ridge patterns on the skin of a fingertip. The functionality of the device is quite unique, as it involves the user's finger to slide a movable glass or transparent platen past the reading elements on the sensor array mounted to the housing, automatically applying a constant pressure on the platen for proper imaging. This provides a positive action whereby the user knows and determines when the scanning cycle commenced and when it was completed.
The main application for the present invention and related prior art is in identification purposes, as fingerprints in human beings remain consistent throughout life and no two are alike. One major application of the device is to be used in conjunction with pattern recognition software means, to identify similarities between fingerprints being scanned and those already scanned and stored on computer systems. Fingerprinting is an established and reliable method of personal identification, and is useful as the basis for a computer controlled security access system. Because fingerprints are generally noisy, due to imperfect inking, smudging etc., it is usually impossible to clean up such images using any thresholding methods. The present device selects features of interest such as the ridge orientations, enabling suppression or elimination of this noise.
Prior art in this field would involve fingerprint reading devices whereby the user places a finger on a fixed glass platen, below which a lens system focuses the illuminated ridge pattern onto a two dimensional area CCD (Charged Coupled Device) chip. Image enhancement means presently used in the art employ prisms or glass surfaces to reveal better ridge patterns. Most fingerprint sensor devices use the prism method whereby a light beam is introduced into a prism from one of its slanted surfaces. This beam is positioned to meet the conditions of total internal reflection at the top surface of the prism. When a finger is placed on top of this surface, the conditions of total internal reflection are no longer met at points of contact with the ridge pattern. Consequently light incident at these points would not be reflected and an image is created of the fingerprint. The image then passes out of the prism at the other slanted surface and is brought into focus on the image pickup element (CCD) with the aid of a lens system. An inherent problem producing a major disadvantage of this prism method is trapezoidal distortion created by unequal optical paths between each point of the fingerprint and the image focusing lens. Other prior art systems which eliminate trapezoidal distortion tend to be quite complicated and expensive in construction. Holographic fingerprint sensors use a laser beam as its light source and consist of a light conducting plate, which is a transparent glass plate with a plain grating type hologram, and a focusing system just under the hologram. Since the sensor uses a planar parallel plate, all the optical paths from each point of a fingerprint to the hologram are equal, creating a defined fingerprint without trapezoidal distortion. These devices do not provide a positive action as the user has no idea at what stage the scanning cycle is at any time.
The main problem associated with most prior art systems for this particular application is the high cost of the lens and CCD chip assembly. Area type CCD's are less reliable than linear CCD's or contact image sensors, and are considerably more expensive. Devices that utilize CCD technology tend to consume a much larger space than contact image sensors, due to the greater focusing distance between the image plane and the CCD.